1. Field of the Invention
The present invention relates to a dampening mechanism, particularly a dampening mechanism which transmits torque between a input rotary member and a output rotary member while dampening a vibration in transmitting the torque.
2. Description of the Related Art
Generally, a dampening mechanism transmits torque between a input rotary member and a output rotary member, while dampening a vibration transmitted from a input rotary member to a output rotary member. One example of such a dampening mechanism is a lock up clutch dampening mechanism (referred to as lock up dampening mechanism hereafter) which is placed inside a torque converter.
A torque converter has three kind of impellers (an impeller, a turbine and a stator) disposed within a housing. The torque converter is a device which transmits torque via hydraulic oil disposed in the housing.
The impeller is fixed to a front cover which is connected to the input rotary member, the front cover partially defining the housing. Torque is transmitted to a power output side via the hydraulic oil flowing from the impeller to the turbine.
The lock up dampening mechanism is positioned between the turbine and the front cover and transmits torque directly from the front cover to the power output member, while connecting the front cover and the turbine mechanically.
Generally, such a lock up dampening mechanism includes a piston which can selectively contact the front cover, a retaining plate fixed to the piston, a torsion spring supported by the retaining plate and a driven plate which is connected to the piston by a torsion spring elastically in a circular direction. The driven plate is fixed to the turbine connected to the output rotary member.
When the lock up dampening mechanism operates, torque is transmitted from the front cover to the piston, and then transmitted to the turbine via the torsion spring. The lock up dampening mechanism also damps a vibration in addition to the transmission of the torque.
A torsion spring is retained by an outer circumferential part of the retaining plate so to prevent outward movement of the torsion spring in a radial direction. The length in a circular direction of the outer circumferential part corresponds to the length of the torsion spring. The outer circumferential part is a bent portion and will hereinafter be referred to as an outer circumferential bending part.
When the lock up dampening mechanism rotates, a centrifugal force is applied to the torsion spring. Hence, the force is applied to the outer circumferential bending part of the retaining plate outward in a radial direction. The thickness of a plate around the outer circumferential bending part must be large enough in order to withstand the stresses associated with the force and to maintain a working life span.
On the other hand, it is desirable for the thickness of the plate to be as small as possible, in order to meet the demand for reduction in weight of a lock up dampening mechanism.